Utility Meets Strict Discharge Permit Limit Using State-Of-The-Art Disinfection System
By Mitch O'Brien & Gary Metz
Keeping up with the demands for environmentally compliant wastewater treatment had posed significant challenges for the City of Baton Rouge and the Parish of East Baton Rouge, La. In recent years, however, the second largest metropolitan area in the state has upgraded its three wastewater treatment facilities and installed state-of-the-art disinfection and dechlorination procedures.
The disinfection system serving the city's 60 mgd Central Wastewater Treatment Plant came on line in 1997. Chlorination and dechlorination are the final stages of treatment at the plant before effluent is discharged into the Mississippi River. The plant must meet a stringent fecal coliform limit: 200 colonies per 100 milliliters on a 30-day geometric mean; and 400 colonies per 100 milliliters on a 7-day geometric mean. Additionally, the plant has a strict chlorine residual limit - 0.78 milligrams per liter, to protect aquatic life near the plant's outfall.
Chlorine is fed from tanks to three gas feed systems which operate under vacuum produced by submersible chemical induction systems in the plant's three chlorine contact tanks. Through the induction units, the chlorine is mixed with effluent from the plant's final settling tanks. The three individual chlorine contact tanks together hold 765,500 million gallons of water. Each tank - 38 feet wide, 62 feet long, and 14.48 feet deep - has a serpentine design that provides four passes, with a detention time of almost an hour at an average design flow of 18.5 mgd and 17 minutes at peak flows of 60 mgd. The chlorine contact tank was an existing structure (bioreactor) that was converted during construction.
Just prior to leaving the contact chambers, the chlorinated effluent is dechlorinated using sulfur dioxide (SO2) metered through three sulfonators and dispersed through submersible mixing units. The dechlorinated effluent is then discharged through a 48-inch outfall pipe into the Mississippi River. An ORP-based system monitors and controls chlorine and sulfur dioxide dosing.
System Components
The triad of components forming any modern chemical feed system includes: 1) feeding, 2) monitoring and control, and 3) diffusion/mixing. Integration of these technologies was key to the effectiveness of chlorination and dechlorination operations at the Central Wastewater Treatment Plant. The necessity to meet a new stringent discharge permit prompted the City of Baton Rouge/Parish of East Baton Rouge Public Works Department, and its engineers, Camp Dresser & McKee, to evaluate, identify and design a new disinfection system.
Feeding
The system's feeding component, and the cornerstone of the plant's disinfection system, consists of a bank of V-2000 chlorinators and sulfonators (USFilter's Wallace & Tiernan Products group). These are floor-mounted units with their instrumentation in modules that form a functional, panel-like array.
The key to feeding accuracy stems from the system's V-notch orifice. The V-notch consists of a precisely grooved plug sliding in a fitted ring. Any position of the plug in the ring results in a specific orifice size and corresponding feed rate, resulting in accurate gas-flow control and repeatability. The units' vacuum regulators have integral double check valves that significantly minimize the possibility of venting gas to the atmosphere.
Diffusion/Mixing
The plant's Water Champ®submersible vacuum chemical induction system (USFilter's Stranco Products group) serves as the disinfection and dechlorination system's diffusion/mixing component and eliminates the need for a conventional injector system for the gas withdrawal process. Chlorine gas and SO2 are pulled from the V-2000 units and delivered directly to the chlorine contact tanks via submersible mixers, thereby eliminating injectors or the need for carrier water. The submersible mixers blast chemical directly into the process stream at velocities up to 60 ft./sec.
Three of the submersible units are installed at the headworks of the contact chambers at the Baton Rouge facility. Positioned in the center of the flow path, the units provide countercurrent mixing at a high velocity gradient across the entire width of the channel. The mixers generate high zone diffusion and a high turbulence region extending across the entire channel as Cl2 is dispersed, providing for a rapid mixing rate. Three units, similarly mounted in the last turn of each contact chamber, disperse SO2 in the same fashion for dechlorination.
Monitoring/Control
Cl2 and SO2 dosing are automatically controlled by a Strantrol®900 High Resolution Redox® (HRR) controller (USFilter's Stranco Products group). Each channel of the controller is programmed with an operator-determined HRR setpoint that corresponds to the disinfection or dechlorination value required to meet discharge limits. The controller automatically goes into alarm mode if HRR readings fall out of the control range.
Chlorine disinfects by oxidizing bacteria, killing the organisms. The disinfection quality is measurable in millivolts (mV). The HRR system responds to changes in demand and the oxidant strength of the chemical used, based on this measurement.
HRR sensors in the chlorine contact chambers respond to numerous reactions, including organic loading increases. As loading increases, the HRR reading drops, signaling the controller to increase the Cl2 dosage accordingly. The same system applies to dechlorination, except the goal is a low HRR reading, indicating that all chlorine has been removed. At the Baton Rouge facility, HRR measurement is direct and continuous, allowing the controller to respond quickly to changes in flow, organic loading and chlorine content.
Conclusion
Leading technologies and effective system integration have led to reliable, automated disinfection and dechlorination at Baton Rouge's Central Wastewater Treatment Plant. The system has worked very well, with minimal attention required other than routine maintenance. The system is currently being integrated with the plant's new SCADA system, enabling operators to fine-tune chlorination and dechlorination operations even further.
About the Author
Mitch O'Brienis laboratory manager and Gary Metzis maintenance supervisor at Baton Rouge's Central Wastewater Treatment Plant, Baton Rouge, LA.